1. Field of the Invention
This invention relates to switched capacitor banks and more specifically to a monolithically integrated switched capacitor bank and fabrication method using micro electro mechanical system (MEMS) technology.
2. Description of the Related Art
Switched capacitor banks are used in microelectronics, especially in the area of RF and Millimeter band communications, to adjust the resonance of a tunable resonator, to alter the transfer function of or to tune a filter, or to match a load impedance of an amplifier. The capacitor bank's performance is evaluated based upon a number of different factors, including its capability of precisely selecting a plurality of capacitance levels over a wide tuning range, linearity, maximum signal and switching frequencies, Q (quality factor), power consumption, isolation, insertion loss, sensitivity to its surrounding environment (vibration, temperature, etc.), and integration. The requirements and relative importance of these different factors depend upon the specific application.
As shown in FIG. 1, a switched capacitor bank 10 includes an array of series-connected switch-capacitor pairs that are connected in parallel between a pair of output terminals 12a and 12b. Ideally the capacitor 14 has no associated resistance or inductance and the switch 16 has zero on-impedance and infinitely high off-impedance. A control circuit 18 switches a digital voltage signal to control the state of each switch 16 in the array to connect and disconnect the capacitors 14 and set the total capacitance seen at the output terminals 12a and 12b to one of a plurality of levels. The total capacitance of the switched capacitor bank is the summation of the capacitances of the switch-capacitor pairs that are connected in parallel. The capacitors can be arranged, for example, such that the switched capacitor bank has a logarithmic behavior; or they can be arranged to have an equal value so that the switched capacitor bank has a linear behavior. More typically, switched capacitor bank 10 consists of capacitors with binary values, i.e., C, 2C, 4C, 8C, etc., which provides a large range of possible capacitance values or a large capacitance ratio.
In one known implementation, discrete mechanical relays are wire bonded to discrete capacitors and controlled by a discrete control circuit. The mechanical relays' metal-to-metal contacts provide high isolation and low insertion losses. The discrete capacitors are formed on glass substrates to minimize their parasitic capacitance. As a result, the switched capacitor bank can be precisely tuned over a large range at very high RF (200 MHz-30 GHz) and millimeter (30 GHz-94 GHz) band frequencies. However, the lack of integration increases both the size and cost to the point that such a system is not feasible in current microelectronic applications.
In another known implementation, the switched capacitor bank and control circuitry are fabricated on a monolithic substrate using conventional planar IC fabrication techniques to form pairs of parallel-plate capacitors, either a metal-oxide-metal or a heavily doped polysilicon-oxide-polysilicon, and FET switches on the surface of the substrate. Integration is essential to minimize cost and to facilitate a large array. The integration of FET switches is a well understood and highly reliable process, in which the resulting devices are very high speed, in excess of 100 MHz, and insensitive to vibrations.
However, the parallel plate capacitors are associated with serial and/or parallel resistors and inductors. In addition, there is a parasitic capacitive coupling between the capacitor and the substrate, which creates an AC path for the high frequency signal to leak to the substrate. Furthermore, the semiconducting nature of the FET switch produces a non-zero insertion loss (typically 1 dB) in the on-state and a less-than-infinite isolation (typically no better than -30 dB) in the off-state. These losses, which increase with signal frequency, limit the capacitor bank's tuning range, level precision, maximum signal frequency, Q, etc. As a result, monolithically integrated switched capacitor banks are limited to applications having frequencies less than 500 MHz or to low-performance applications at higher frequencies.
Consequently, RF transceivers that operate at frequencies above approximately 1 GHz use multiple chip sets that are fabricated with different technologies to optimize the capacitors, the FET switches, and the control circuitry, respectively, to get satisfactory insertion loss and isolation. For example, the capacitors may be formed on a glass substrate, the FET switches on a gallium arsenide substrate, and the control circuitry on a silicon substrate.
MEMS have been used in such applications as pressure sensors, accelerometers, and nozzles, and have been proposed for use in RF telecommunications systems. In particular, a number of different types of MEMS switches have been developed. Petersen, K. "Micromechanical Membrane Switches on Silicon," IBM J. Res. Develop., vol. 23, 1979, pp. 376-385 discloses a chemical etching process for fabricating a mechanical switch, which is sensitive to vibrations and has poor insertion loss and isolation. Gretillat et al, "Electrostatic Polysilicon Microrelays Integrated with MOSFETs," in proceedings of Micro Electro Mechanical Systems Workshop, 1994, pp. 97-101 describes a switch for use in a automated testing applications, which exhibits large insertion loss and high frequency capacitive coupling to its polysilicon cantilever arm in its off-state. Yao et al. "A Surface Micromachined Minature Switch for Telecommunications Applications with Signal Frequencies from DC up to 4 GHz" In Tech. Digest, Transducer-95, Stockholm, Sweden, Jun. 25-29, 1995, pp. 384-387 describes a switch for use in RF telecommunications that uses electrostatic actuation to control a silicon dioxide cantilever arm to open and close a signal line, and has an electrical isolation of -50 dB and an insertion loss of 0.1 dB at 4 GHz.
As a type of mechanical relay, one would not normally integrate MEMS switches with microelectronic circuitry such as capacitors, but would instead use them in discrete multi-chip sets. The historical concern is that mechanical parts in general, and relays in particular, are not reliable. In particular, repeated cycles stress the cantilever arm and damage the mechanical contact, which in turn can increase the contact resistance and cause the switch to become welded shut. Furthermore, the switch's cantilever arm is typically very sensitive to vibrations that may cause false on-states. Thus, MEMS switches would be provided as a discrete chip that could be designed to try to reduce these problems, and which could be replaced if the chip should fail. Furthermore, the discrete capacitors can be formed on glass substrates, rather than semi-insulating substrates such as GaAs, which minimizes the parasitic coupling to the substrate.